Sound muffling material and method of making thereof

ABSTRACT

A sound muffling material for use in combustion engine exhaust mufflers is provided. The material includes volumized fibers retained in compressed form by a material of lower softening temperature than the fibers. When the material is heated by exhaust gases, the material of lower softening temperature is softened allowing the compressed fibers to expand. The fibers may be formed into a knitted fabric retained by a sacrificial thread. The material may be used to support catalyst bricks.

The present invention relates to a sound muffling material. The materialis intended particularly although not exclusively for use in mufflersand silencers fitted to internal combustion engine exhausts.

Exhaust mufflers generally include a sound muffling material, usuallyglass fibers. This material acts to attenuate sounds transmitted throughthe exhaust system. The fibers are usually disposed in at least a partof the muffler. The fibers generally fill a part of the muffler to acertain density to achieve an effective muffling effect. The fibers areusually in a volumized form.

In one existing arrangement an exhaust muffler includes a cylindricalsteel body, usually referred to as a box, in which there is disposedcoaxially a perforated steel tube. The perforated steel tube is mountedon annular end caps which are affixed to opposite ends respectively ofthe cylindrical body by welding or crimping. A muffling material,usually glass fibers, is disposed in the annular region between theperforated steel tube and the muffler body. In use exhaust gases aredirected through one end cap, along the perforated tube, and out theopposite end cap.

Mufflers of this type are assembled in one of two common ways. Generallythe muffler is assembled by attaching one end cap to support theperforated tube. In one method volumized continuous filament glassfibers are injected, through the open end of the muffler body, into theannular region between the perforated tube and muffler body usingspecialist equipment. In another method a glass fiber needlefelt fabricis provided wrapped around a cardboard tube, or former, of a similardiameter to the perforated metal tube in the muffler. The cardboard tubeis positioned above the perforated tube and the cylinder of needlefeltfabric slid off the cardboard tube and onto the perforated tube.

There are a number of problems associated with both the above describedmethods. The equipment used to inject continuous filament fibers isexpensive and therefore limits the number of mufflers that can beproduced simultaneously at reasonable cost. When fibers are provided inthe form of a needlefelt they are not necessarily continuous filamentfibers. As such, when the muffler is used some fibers may pass throughthe perforated tube and into the flow of exhaust gases. This isundesirable as the effect of the muffler will be diminished and theescaping fibers may cause problems in the remainder of the exhaustsystem. Also, sliding a cylinder of needlefelt fabric onto a perforatedtube is also inconvenient as the fabric tends to snag on the cut end ofthe perforated tube. Further, the cardboard tubes present a wastemanagement problem. Often, the tubes are re-used which necessitatesreturning the tubes to the supplier, increasing transport costs.

By far the most significant drawback with conventional methods offilling mufflers with fibers is that where the fibers are loose,especially in the case of the preferred continuous filament fibers, andthe muffler is filled with the required density of fibers this presentsproblems when the end cap is attached to the muffler. Where fibers strayout of the muffler body they may become trapped between the muffler bodyand the end cap. This adversely affects the quality of the join betweenthe end cap and muffler body both when the end cap is attached bywelding and crimping. It is therefore essential that before the end capis attached to a muffler body the fibers disposed in the body arecarefully moved from the region of the join. This is tedious and timeconsuming.

Another type of muffler is the clam shell type, which includes twoportions which are crimped or welded together to form a complete unit.Mufflers of this type are produced in a variety of shapes and sizes, ingeneral, however, each half is relatively shallow. The clam shell typeof muffler cannot be easily filled with fibers using the above describedmethods as the fibers easily escape. Instead, short fibers are providedpacked in, or continuous filaments injected into, perforated polythenebags. A bag of fibers is placed into one half of a clam shell mufflerand the second half is welded or crimped to the first half. In use, hightemperature exhaust gases cause the polythene bags to disintegrate,releasing the fibers. Again, there are problems associated with thistechnique. Firstly, the bags tend to be bulky in order to provide thecorrect density of fibers to fill the muffler. This makes joining thetwo halves of the muffler difficult. Secondly, where the bag is filledwith short filament fibers problems are experienced with the fibersescaping from the muffler in use, as described above.

EP 0434895A discloses a silencer for an internal combustion enginecomprising a hollow housing containing a web of fibers and a pipeextending therethrough. The web of fibers is confined by a plastics filmand there is a substantial clearance space between the film and thehousing. When the silencer has been connected with an internalcombustion engine and is subjected to flow of hot exhaust gases from theengine the plastics film inside the silencer is destroyed so that it nolonger confines the web of fibers.

DE 3827863A discloses an exhaust gas purification device which includesa resilient support mat. The resilient support mat is surrounded by acovering sheet such that it is compressed. In one arrangementoverlapping marginal areas of the covering sheet are fixed together withblobs of adhesive which melts on heating to permit separation of theedges and expansion of the support mat.

WO 91/19082 discloses a protective material for a catalytic convertorblock comprising a pad of fibrous material in an envelope of non-woventextile material. The envelope has its depth reduced in at leastlocalized areas or positions by drawing together of opposing faces bystitching.

It is an object of the present invention to provide a convenient methodof filling a muffler with fibers, particularly to enable continuousfilament fibers to be easily used in clam shell type mufflers.

According to a first aspect of the present invention there is provided asound muffling material comprising volumised continuous filament fibersretained in a compressed state in the form of a knitted or woven fabricwith a density of at least 200 kg/m³ by a material of lower softeningtemperature than the fibers, arranged so that the material of lowersoftening temperature will release the fibers when heated.

According to a second aspect of the present invention there is provideda sound muffling material comprising volumised continuous filamentfibers retained in a compressed state in the form of a knitted or wovenfabric with a density of at least 200 kg/m³ by a material which breaksdown at a lower temperature than the fibers, arranged so that thematerial of lower softening temperature will release the fibers whenheated.

According to a third aspect of the present invention there is provided amethod of making a sound muffling material comprising the steps ofproviding continuous filament fibers, volumising the fibers, providing amaterial with a lower softening temperature than the fibers, compressingthe volumised fibers and retaining the volumized in a compressed stateby means of the material of lower softening temperature by forming thevolumized fibers into a knitted or woven fabric with a density of atleast 200 kg/m³.

According to a fourth aspect of the present invention there is provideda method of making a sound muffling material including the steps ofproviding continuous filament fibers, volumising the fibers, providing amaterial with a lower breakdown temperature than the fibers, compressingthe volumized fibers and retaining the volumized in a compressed stateby means of the material of lower breakdown temperature by forming thevolumised fibers into a knitted or woven fabric with a density of atleast 200 kg/m³.

According to a fifth aspect of the present invention there is provided amethod of filling an exhaust muffler with fibers including the steps ofplacing a material according to either of the first or second aspects ofthe present invention into an exhaust muffler and heating the materialso as to release the fibers.

According to a sixth aspect of the present invention there is provided amethod of mounting an exhaust catalyst brick comprising the steps ofwrapping the brick in a material according to either of the first orsecond aspects of the present invention and heating the material so asto release the fibers.

The material is preferably adapted for insertion into an internalcombustion engine exhaust muffler, including both domestic andcommercial vehicles as well as industrial applications, for instancesilencers used on gas turbine installations and during jet enginetesting. The material may also be used for catalyst brick support inexhaust systems.

The fibers are preferably heat resistant and may include silica, glass,mineral or basalt man made fibers. The fibers preferably comprisee-glass (electrical glass) fibers. The fibers are also preferablyresistant to exhaust gases.

The fibers are preferably resistant to thermal breakdown at temperaturesup to 500° C., more preferably 1000° C., still more preferably 1100° C.or higher.

The average length of the fibers is preferably greater than 400 mm.

The fibers may be volumised by the process known as air texturising orvolumizing.

The fibers may be volumized by using conventional compressed airoperated volumizing equipment to separate the filaments inmulti-filament strands or yarns, for example multiple fibre roving. Thevolume occupied by the fibers is preferably increased by at least afactor of ten. The fibers may also be texturized, again usingconventional equipment, for example air-jet texturizing equipment.

The volumized heat resistant fibers are preferably retained, when incompressed form, so as to minimize their volume.

The volumized heat resistant fibers are preferably retained by anorganic or synthetic material with a softening/melting point of lowertemperature than that of exhaust gases, more preferably less than 200°C., still more preferably below 150° C. The retaining materialpreferably includes a fiber, for example a nylon polypropylene,polyethylene or polyester fiber. It is to be understood, however, thatnatural materials and fibers which breakdown at temperatures below thesoftening or breakdown temperature of the heat resistant fibers could beused, for example cotton fibers.

More generally the heat resistant fibers and retaining material arepreferably chosen so that in use, for example in an exhaust muffler, thehigh temperature exhaust gases cause the retaining material to breakdownto release the heat resistant fibers. This allows mufflers and otherequipment to be easily assembled with heat resistant fibers in acompressed form. As such the fibers take up a minimum of volume thisovercomes the problem of stray fibers interfering with the assembly ofthe muffler and the difficulty associated with the insertion of bulkyfibers into a muffler. When the muffler is first used the fibers arereleased and expand to fill the muffler in a desired manner.

In a preferred arrangement the heat resistant fibers are formed into acrochet or rochel knit fabric, retained by a lower melting point thread,for example a ‘sacrificial’ catch thread. The fabric may however takeother forms, for example a woven fabric where the warp and weft includerespectively heat resistant and heat softening fibers, or vice versa.Braided, twisted or netted methods of manufacture may also be used.

Fabrics according to the present invention may be configured so thatupon the melting/breakdown of the retaining material the fabric expandsin a predetermined manner. For example a strip of fabric may be arrangedso that it will expand mainly in length and thickness but less so inwidth. This is a useful feature where the fabric is used in acylindrically bodied muffler.

When the arrangement of fibers comprises a fabric it is preferable thatthat fabric has a density of at least 60 kg/m³, more preferably 200kg/m³ and still more preferably 400 kg/m³, in compressed form, beforesoftening/breakdown of the retaining threads.

When the arrangement of fibers is a fabric this may be producedcontinuously and cut into pieces of desired length. It is preferablethat the ends of the fabric are secured to prevent fraying and prematureexpansion, for example by taping the ends of the fabric or using athread lock adhesive. It is preferred that any tape or adhesive has asoftening/thermal breakdown temperature of a similar order to theretaining material and in any event lower than that of the heatresistant fibers.

Portions of material of the present invention may be packed in plasticbags to aid handling. Such bags preferably breakdown on exposure to heatin exhaust systems.

The present invention provides an improved method of and mufflingmaterial for filling exhaust mufflers. The method dispenses with theneed for the use of either expensive equipment or for cardboard formersor other packaging. As the fibers are provided in compressed form theytake up less volume and are therefore considerably easier to insert intomuffler boxes. As the fibers are retained they are also less likely tointerfere with the closing of muffler boxes by crimping or welding.

Forming the fibers into a fabric provides the ability to controlaccurately the density of infill of muffler boxes and the like. Theyalso allow a much higher overall fill density of fibers to be achievedthan with conventional materials and methods.

Where continuous filament fibers are provided this reduces the tendencyof fibers to escape into an exhaust system.

Fabrics may also be used as a catalyst support mat for catalyst bricksupport. Catalyst bricks cannot be welded. Fabrics can be used to retaincatalysts in exhaust systems by wrapping the catalyst brick in a fabric,the wrapped catalyst brick is then placed in a part of an exhaustsystem, often similar to a muffler box.

Where fabrics according to the present invention are employed they canbe arranged to expand on initial heating to firmly secure a catalystbrick in place and take account of the differential expansion of thecatalyst brick and housing. This minimises any movement of the catalystbrick, caused, for example, by vibration of an exhaust system, and soprolongs catalyst life.

In order that the invention may be more clearly understood there axe nowdescribed embodiments thereof, by way of example and with reference tothe accompanying drawings in which:

FIG. 1 shows a plan view of a fabric;

FIG. 2 shows a cross-section through the fabric illustrated in FIG. 1,taken along the line ll-ll;

FIG. 3 shows a similar view to FIG. 2 of a similar fabric in expandedform;

FIG. 4 shows a transverse cross-section through a cylindrically bodiedexhaust muffler, containing a fabric in compressed form;

FIG. 5 shows a longitudinal cross-section through a similar muffler tothat of FIG. 4, containing a fabric in expanded form;

FIG. 6 shows an exploded perspective view of a clam shell type muffler;

FIG. 7 shows a transverse cross-section through an exhaust catalystcontaining a catalyst brick supported by a fabric; and

FIG. 8 shows a view similar to FIG. 7 where the fabric has beenexpanded.

Referring to FIGS. 1 and 2 there is shown a rochel knit fabriccomprising e-glass fibers 1 retained by a polyethylene catch thread 2.The e-glass fibers 1 have been volumized, but are retained in compactform by the catch thread 2. The e-glass fibers 1 are in the form ofcontinuous filament roving. Other knit or weave styles may be usedprovided that they enable volumised glass fibers to be retained by afiber of lower softening point.

The fabric has been cut from a continuous length and ends 3 and 4 havebeen bound with a plastic tape to prevent fraying of the fibers. Thefabric has a density of approximately 600 kg m³.

In this form the portion of fabric may be wrapped in a polythene bag toreduce exposure of persons handling the material to the glass fiberswhich may act as a skin irritant.

The density of the fabric may be increased or decreased as required byaltering the number of catch threads.

FIG. 3 shows a similar fabric to that of FIGS. 1 and 2 following heatingof the fabric to a temperature sufficient to soften the catch threadssufficiently to enable the volumized glass fibers to break free from thecatch threads. When the fabric is so heated, as would occur in anexhaust system, it expands and considerably increases in volume. FIG. 3shows expanded e-glass fibers 5, which have returned to their volumizedform.

FIG. 4 shows a transverse cross-section through a cylindrical typeexhaust muffler. The muffler includes a steel outer casing or box 6 andan inner perforated steel tube 7 which is disposed coaxially within thebox 6.

For mufflers of this type to work effectively it is necessary that theannular region, generally indicated at 8 between the box 6 and theperforated tube 7 is filled with a relatively uniform density of heatresistant fibers.

A method of filling this region according to the invention isillustrated by FIGS. 4 and 5. FIG. 4 shows a fabric 9, similar to thatillustrated in FIGS. 1 and 2, which has been placed into the annularregion 8. The fabric fills only a small proportion of the volume of theannular region 8 and is therefore easy to insert. Further, as the fabricis compact and does not have fraying ends or stray fibers the fabricdoes not interfere with the open ends of the box 6 during its assembly.

FIG. 5 shows a transverse cross-section through a similar muffler tothat illustrated in FIG. 4. Again, the muffler comprises a box 10 and aninner perforated tube 11. End caps 12 and 13 are affixed to oppositeends respectively of the box 10 and tube 11. The end caps 12, 13 closethe box 10 and serve to support the tube 11 coaxially within the box 10.The end caps 12 and 13 are secured to both the box 10 and the tube 11 bywelding. In use, the muffler is installed into an exhaust system byconnecting pipes, shown partially at 14 and 15 to openings in the endcaps so as to direct exhaust gases through the perforated tube 11.

FIG. 5 shows the effect of heating a fabric, similar to that indicatedas 9 in FIG. 4, which is disposed in the annular region between the box10 and tube 11. On heating the catch threads of the fabric have softenedallowing the fabric to expand thereby filling the annular regionsurrounding the perforated tube 11 with glass fibers 16. As the fabricis only heated when the muffler is assembled the expansion of the fibersdoes not interfere with the assembly of the muffler and in particularwith the attachment of the end caps 12, 13 to both the box 10 andperforated tube 11.

FIG. 6 shows an alternative clam shell type of muffler, the body ofwhich is formed from two parts, 17 and 18. Part 17 includes a perforatedtube 19 connecting apertures 20 and 21 formed in part 17.

During assembly of this type of muffler it is necessary to fill the bodyof the muffler surrounding the perforated tube with a muffling materialand then join the two portions of the muffler 17 and 18 by, for example,crimping or welding them together. It is difficult when assembling thistype of muffler to keep the bulky muffling material away from theregions of the muffler body which must be joined by welding or crimping.This problem is solved by the present material and method. For example afabric, such as that illustrated in FIGS. 1 and 2, may be placed intopart of the muffler 17. The fabric is compact and will occupy only asmall proportion of the muffler's volume and therefore will notinterfere with the joining of the two portions of the muffler 17 and 18together. When the muffler has been assembled and installed in anexhaust system and exhaust gases are fed through the muffler this willheat the fabric causing it to expand and evenly fill the muffler withfibers as required.

The present material and method provide a convenient and economical wayof filling exhaust mufflers with fibers. Where cylindrical mufflers areconcerned the need is nagated for specialist equipment for injectingfibers or for cardboard formers used to slide pre-formed needlefeltfabrics into the muffler. In particular the present material and methodallow continuous filament fibers to be conveniently inserted intomufflers. Continuous filament fibers are preferred as they are lesslikely to escape from the muffler through perforations in the pipeconducting exhaust gas flow. Where clam shell type mufflers areconcerned the present invention conveys a considerable advantage in thaton insertion the muffling fibers are in compressed form and do notinterfere with the assembly of the muffler. Only when the muffler hasbeen completed and is used for the first time are the fibers distributedthroughout the muffler body. Present methods do not allow the filling ofclam shell type mufflers with continuous filament fibers.

Referring to FIGS. 7 and 8 another use for fabric according to thepresent invention is for catalyst brick support. FIGS. 7 and 8 show atransverse cross-section through a cylindrical box 22 in which acatalyst brick 23 is installed. FIG. 7 shows the arrangement asassembled. The catalyst brick 23 is supported by a rochel knit glassfiber fabric 24, with a nylon catch thread. The fabric 24 is wrappedaround the brick 23.

FIG. 8 shows the same arrangement following heating of the fabric bypassing exhaust gases through the arrangement. On heating, the catchthreads of the fabric are softened allowing the glass fibers, which havepreviously been volumized, to expand filling the region surrounding thecatalyst brick 23 with fibers 26. Following expansion of the fabric thefibers 26 hold the catalyst brick 23 firmly within the box 22 preventingmovement of the catalyst brick 23. The fibers 26 also serve to insulatethe catalyst brick 23 from the box 22. This allows the catalyst brick 23to rapidly achieve its working temperature.

The above embodiments are described by way of example only and manyvariations are possible without departing from the invention.

1. A sound muffling material comprising volumized continuous filament fibers retained in a compressed state in the form of a knitted or woven fabric with a density of at least 200 kg/m³ 60 kg/m³ by a material of lower softening temperature than the fibers, arranged so that the material of lower softening temperature will release the fibers when heated.
 2. A sound muffling material comprising volumized continuous filament fibers retained in a compressed state in the form of a knitted or woven fabric with a density of at least 200 kg/m³ 60 kg/m³ by a material which breaks down at a lower temperature than the fibers, arranged so that the material of lower softening temperature will release the fibers when heated, wherein the average length of the fibers is greater than 400 mm.
 3. The sound muffling material according to claim 1 wherein the fibers comprise glass fibers.
 4. The sound muffling material according to claim 1 wherein the fibers are resistant to thermal breakdown up to 1000° C.
 5. The sound muffling material according to claim 1 wherein the fibers are retained by a material selected from the group consisting of nylon, polypropylene, polyethylene or polyester.
 6. The sound muffling material according to claim 1 wherein the fibers are retained by a material with a softening or melting temperature below 200° C.
 7. The sound muffling material according to claim 1 wherein the material of lower softening temperature than the fibers is itself a fiber.
 8. The sound muffling material according to claim 1 wherein the material of lower softening temperature than the fibers is a sacrificial catch thread.
 9. The sound muffling material according to claim 1 wherein the fabric has a density of at least 400 kg/m³ in compressed form.
 10. The sound muffling material according to claim 1 wherein the fabric has a density of the order of 600 kg/m³ in compressed form.
 11. The A method of filling an exhaust muffler with fibers comprising the steps of: providing a material including volumized continuous filament fibers retained in a compressed state in the form of a knitted or woven fabric with a density of at least 200kg/m³ 60 kg/m³ by a material of lower softening temperature than the fibers, arranged so that the material of lower softening temperature will release the fibers when heated; placing the material into an exhaust muffler; and heating the material so as to soften the retaining material to release the fibers, wherein the average length of the fibers is greater than 400 mm.
 12. A method of filling an exhaust muffler with fibers, comprising the steps of: providing a material including volumized continuous filament fibers retained in a compressed state in the form of a knitted or woven fabric with a density of at least 200kg/m³ 60 kg/m³ by a material which breaks down at a lower temperature than the fibers, arranged so that the material of lower softening temperature will release the fibers when heated; placing the material into an exhaust muffler; and heating the material so as to cause the retaining material to breakdown to release the fibers, wherein the average length of the fibers is greater than 400 mm.
 13. A method of mounting an exhaust catalyst brick, comprising the steps of: providing a material including volumized continuous filament fibers retained in a compressed state in the form of a knitted or woven fabric with a density of at least 200kg/m³ 60 kg/m³ by a material of lower softening temperature than the fibers, arranged so that the material of lower softening temperature will release the fibers when heated: wrapping the brick in the material; and heating the material so as to soften the retaining material to release the fibers.
 14. A method of mounting an exhaust catalyst brick, comprising the steps of: providing a material including volumized continuous filament fibers retained in a compressed state in the form of a knitted or woven fabric with a density of at least 200 kg/m³ 60 kg/m³ by a material which breaks down at a lower temperature than the fibers, arranged so that the material of lower softening temperature will release the fibers when heated: wrapping the brick in the material; and heating the material so as to cause the retaining material to breakdown to release the fibers.
 15. The method according to claim 13 further comprising the step of inserting the wrapped brick into a box, before heating the material.
 16. A method of making a sound muffling material comprising the steps of providing continuous filament fibers, volumizing said fibers, providing a material with a lower softening temperature than the fibers, compressing said volumized fibers and retaining said volumized in a compressed state by means of the material of lower softening temperature by forming said volumized fibers into a knitted or woven fabric with a density of at least 200 kg/m³ 60 kg/m³, wherein the average length of the fibers is greater than 400mm.
 17. A method of making a sound muffling material comprising the steps of providing continuous filament fibers, volumizing said fibers, providing a material with a lower breakdown temperature than the fibers, compressing said volumized fibers and retaining said volumized in a compressed state by means of the material of lower breakdown temperature by forming said volumized fibers into a knitted or woven fabric with a density of at least 200 kg/m³. 60 kg/m³, wherein the average length of the fibers is greater than 400 mm.
 18. The method according to claim 16 wherein the fibers are provided in the form of multi-filament strands.
 19. A The method according to claim 18 wherein the multi-filament strands comprise multiple fibre roving.
 20. The method according to 16 wherein the fibers are volumized using compressed air operated volumizing equipment.
 21. The method according to claim 16 wherein the volumizing step increases the volume occupied by the fibers by at least a factor of ten.
 22. The method according to claim 14 further comprising the step of inserting the wrapped brick into a box, before heating the material.
 23. The method according to claim 17 wherein the fibers are provided in the form of multi-filament strands.
 24. The method according to claim 17 wherein the fibers are volumized using compressed air operated volumizing equipment.
 25. The method according to claim 17 wherein the volumizing step increases the volume occupied by the fibers by at least a factor of ten.
 26. The sound muffling material according to claim 2 wherein the fibers comprise glass fibers.
 27. The sound muffling material according to claim 3 wherein the average length of the fibers is greater than 400 mm.
 28. The sound muffling material according to claim 2 wherein the fibers are resistant to thermal breakdown up to 1000° C.
 29. The sound muffling material according to claim 2 wherein the fibers are retained by a material selected from the group consisting of nylon, polypropylene, polyethylene or polyester, and combinations thereof.
 30. The sound muffling material according to claim 2 wherein the fibers are retained by a material with a softening or melting temperature below 200° C.
 31. The sound muffling material according to claim 2 wherein the material of lower softening temperature than the fibers is itself a fibre fiber.
 32. The sound muffling material according to claim 2 wherein the material of lower softening temperature than the fibers is a sacrificial catch thread.
 33. The sound muffling material according to claim 2 wherein the fabric has a density of at least 400 kg/m³ in compressed form.
 34. The sound muffling material according to claim 2 wherein the fabric has a density of the order of 600 kg/m³ in compressed form.
 35. A sound muffling material comprising volumized continuous filament fibers retained in a compressed state in the form of a knitted or woven fabric with a density of at least 200kg/m³ 60 kg/m³ by a material of lower softening temperature than the fibers, arranged so that the material of lower softening temperature will release the fibers when heated, wherein the average length of the fibers is greater than 400 mm.
 36. The sound muffling material according to claim 1 wherein the fabric has a density of at least 200 kg/m³ in compressed form.
 37. The sound muffling material according to claim 2 wherein the fabric has a density of at least 200 kg/m³ in compressed form.
 38. The sound muffling material according to claim 11 wherein the fabric has a density of at least 200 kg/m³ in compressed form.
 39. The sound muffling material according to claim 12 wherein the fabric has a density of at least 200 kg/m³ in compressed form.
 40. The sound muffling material according to claim 13 wherein the fabric has a density of at least 200 kg/m³ in compressed form.
 41. The sound muffling material according to claim 14 wherein the fabric has a density of at least 200 kg/m³ in compressed form.
 42. The sound muffling material according to claim 16 wherein the fabric has a density of at least 200 kg/m³ in compressed form.
 43. The sound muffling material according to claim 17 wherein the fabric has a density of at least 200 kg/m³ in compressed form.
 44. The sound muffling material according to claim 35 wherein the fabric has a density of at least 200 kg/m³ in compressed form. 